Radar system and components therefore for transmitting an electromagnetic signal underwater

ABSTRACT

A radar system is shown to include a transmitter for transmitting a pulsed electromagnetic signal having a frequency less than 500 Hz and a receiver for receiving a (scattered) reflected signal provided from the pulsed electromagnetic signal (scattered) reflected from an anomaly below the surface of the water. The radar system further includes a switch for inhibiting the receiver from receiving a reflected signal provided from the pulsed electromagnetic signal reflected from the surface of the water and a signal processor for controlling interoperability of the transmitter, the receiver and the switch. With such an arrangement, a radar system is provided for detecting anomalies such as a wake of a moving vessel (if and when this produces a conductivity anomaly), or a plume of oil beneath the surface of water.

This invention was made with Government support under Contract No.N61533-90-C-0080 awarded by the Department of the Navy. The Governmenthas certain rights in this invention.

BACKGROUND OF THE INVENTION

This invention relates to radar systems and more particularly to a radarsystem and components therefore for transmitting an electromagneticsignal from above the surface of water to below the surface of water todetect anomalies below the surface of water.

As is known in the art, it is often desirable to detect the present ofobjects under the surface of water. Typically, conventional underwaterdetection systems operate on acoustic principles. That is, acousticenergy is transmitted in the water medium and echo return signals arereceived and processed to determine the presence of objects. However,acoustic signal propagation requires a transmitting transducer and areceiving transducer be disposed in the water medium. If an underwaterdetection system is utilized from an aircraft (i.e. airplane,helicopter, etc.), then the transmitting transducer and the receivingtransducer must be suspended from a cable or a towline attached to theaircraft with the transmitting transducer and the receiving transducerdisposed below the water's surface. Unfortunately, such an arrangementreduces the mobility of the aircraft and the rate of searchingcapability of the system. Thus, it is desirable to not requiretransducers disposed in the water for an underwater detection systemutilized from an aircraft.

An alternative technique for detecting objects is utilizing a radarsystem transmitting electromagnetic signals. Electromagnetic signalspropagate effectively in air and couple suitably to sea water.Unfortunately, electromagnetic signals do not propagate well in a watermedium wherein the signals are subjected to a high rate of attenuation.In sea water, an electromagnetic signal is typically attenuated 8.68 dBper skin depth. Skin depth is frequency dependent and typically with afrequency of 1 Hz, the skin depth is 252 meters, with a frequency of 100Hz, the skin depth is 25.2 meters and with a frequency of 10 KHz, theskin depth is only 2.52 meters. If a signal having a frequency of 100 Hzis propagating through sea water a distance of 1,000 meters, then thesignal would have travelled a distance of approximately 40 skin depthswhich equals 347 dB of attenuation. From the latter, it should beappreciated that it is desirable to use relatively low frequencyelectromagnetic signals if the signal is required to penetrate thesurface of the water a significant amount of distance.

An electric antenna for transmitting an electromagnetic signal istypically a multiple of an one-half wavelength or variant thereof suchthat the electromagnetic signal is resonant with the antenna. Atfrequencies approaching 100 Hz, a signal has a wavelength in air so longthat an electric antenna is not practical for use in an aircraft. Analternative antenna for a signal with a frequency of 100 Hz, is amagnetic dipole antenna for transmitting the electromagnetic signal. Onesuch antenna is described in a publication entitled "Air/UnderseaCommunication at Ultra-Low-Frequencies Using Airborne Loop Antennas" byA. C. Fraser-Smith, D. M. Bubenik and O. G. Villard, Jr. wherein a loopantenna is described for use in an airplane. However, such an antenna islarge and spanned the entire aircraft. Also the antenna had a largeinductance, which may not be a problem in some instances, but istypically not desirable.

SUMMARY OF THE INVENTION

With the foregoing background in mind, it is an object of this inventionto provide a radar system for transmitting a signal underwater anddistinguishing an echo return signal of interest from a return signalcaused from the water surface.

Another object of this invention is to provide a radar system capable ofdetecting an anomaly, or conductivity disturbance, beneath the surfaceof water.

Another object of this invention is to provide a radar system capable ofdetecting a plume of oil disposed beneath the surface of water.

Another object of this invention is to provide a radar system capable ofdetecting a wake from a vessel moving beneath the surface of water, ifand when the motion of the vessel causes a conductivity perturbation.

Still another object of this invention is to provide a magnetic dipoleantenna capable of providing a magnetic moment of approximately 10⁶ to10⁷ Am².

In accordance with a further aspect of the present invention, an antennafor transmitting electromagnetic signals having a frequency less than500 Hz includes a beam of dielectric material and a conductive sheetwound in a helical pattern around the beam of dielectric material. Theantenna further includes a core fabricated from a silicon and ironalloy, the core disposed between the beam of dielectric material and theconductive sheet. With such an arrangement, a magnetic dipole antenna isprovided capable of providing a magnetic moment of approximately 10⁶ to10⁷ Am².

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this invention, reference is nowmade to the following description of the accompanying drawings, wherein:

FIG. 1 is a simplified block diagram of a radar system mounted in anaircraft in a typical engagement according to the invention;

FIG. 1A is a diagram of a plot of the transmitted signal transmitted bythe radar system of FIG. 1;

FIG. 1B is a diagram of a plot of the received signals received by theradar system of FIG. 1;

FIG. 2 is a simplified block diagram of the transmitter of the radarsystem of FIG. 1;

FIG. 3 is an isometric view of the antenna of the radar system of FIG.1; and

FIG. 3A is an enlarged isometric view of a portion of the antenna of theradar system of FIG. 1;

FIG. 4 is an exemplary scene of an airplane travelling over a targetalong with a displayed waveform corresponding to the detection of thetarget by the radar system of FIG. 1; and

FIG. 5 is a diagram of a radar system mounted in an aircraft in analternative engagement according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a radar system 10 mounted within an aircraft110 is shown to include a transmitter 12 for transmittingelectromagnetic signals. An output of the transmitter 12 is connected toa switch box 14 which is coupled to a transmit antenna 20, here ahorizontal magnetic dipole. The radar system further includes a receiveantenna 18 connected to a receiver 16 and a digital signal processor(DSP) 22 which provides control signals to the transmitter 12, theswitch box 14 and the receiver 16. The DSP 22 is also connected to thereceiver 16 to receive signals from the receiver 16 and also isconnected to a video display 24. An inertial measurement unit (INU) 26is connected to the DSP 22 to provide the DSP 22 with navigationalinformation (i.e. height above surface, etc.).

The transmitter 12 is adapted to provide a pulsed electromagnetic signalhaving a frequency below 500 Hz in a manner as described in more detailhereinafter. Suffice it to say here that the electromagnetic signal fromthe output of the transmitter 12 is coupled to the antenna 20 by theswitch box 14 in response to a control signal from the DSP 22. Theswitch box 14 is adapted, in response to the control signal from the DSP22, to either connect or disconnect the transmitter 12 with the antenna20. The antenna 20 will, when connected to the transmitter 12, emanatean electromagnetic signal 122 toward a surface 114 of sea water 112. Aportion of the electromagnetic signal 122 is reflected by the surface114 of the sea water 112 as a first reflected signal 124 and a portionof the electromagnetic signal 122 will enter below surface 116 of thesea water 112 as an underwater signal 126. The underwater signal 126will propagate through the sea water 112 until striking a disturbance120 wherein a portion of the underwater signal 126 is reflected as asecond reflected signal 128. The second reflected signal 128 willpropagate through the sea water 112 until reaching the surface 114wherein the second reflected signal will enter the air and propagatetoward the aircraft 110. A portion of the second reflected signal 128will strike the antenna 18. The antenna 18 will capture that portion ofthe second reflected signal 128 incident thereon and couple the signalto the receiver 16. The receiver 16 processes such incident signal todetect the presence of objects, for example, a submarine 118 or here thedisturbance 120.

As described hereinabove, electromagnetic signals can propagate inwater, however the depth of penetration is inversely related to thefrequency of the electromagnetic signal. More particularly, in seawater, an electromagnetic signal is attenuated at a rate of 8.68 dB perskin depth. Typically, a signal having a one-way loss greater than 25 dBin the sea water is too weak to provide useful detection information bythe time it is processed by the radar system 10. Thus, the one-way depthof penetration for satisfactory return signals is approximately threetimes the skin depth of the electromagnetic signal (we assume here thatwe have available enough reserve in Signal-to-Noise ratio to afford atotal round trip of 6 skin depths). Knowing the latter, a one Hz signalhaving a skin depth of approximately 252 meters can satisfactorilypropagate to a depth of 756 lot)) meters, whereas a 100 Hz signal havinga skin depth of approximately 25.2 meters can satisfactorily propagateto a depth of approximately 75 meters. From the latter, it should now beappreciated that it is desirable for the electromagnetic signal to havean extra low frequency, preferably in the range from 40 Hz to one Hz.Basic parameters of propagation of electromagnetic waves having afrequency of one, ten, 75 and 100 Hz are shown in Table I.

                                      TABLE I                                     __________________________________________________________________________                        Skin Depth &                                                                         Phase  Two-Way                                     Frequency                                                                           Skin Depth &                                                                         Wavelength                                                                           Attenuation                                                                          Velocity                                                                             Phase Delay                                 (Hz)  (meters)                                                                             (meters)                                                                             (dB)   (meters/sec)                                                                         (msec/100 m)                                __________________________________________________________________________     1    252.0  1,583  8.68    1,583 126                                         10    79.7   500    8.68    5,000 40                                          75    29.1   182.8  8.68   13,710 14.6                                        100   25.2   158.3  8.68   15,830 12.6                                        __________________________________________________________________________

In view of the above, as the submarine 118 is travelling below thesurface 114, a disturbance 120 (i.e. a wake) is left behind thesubmarine 118. It should be noted that, if the sea water is suitablystratified, the wave 120 is detectable well after the submarine 118passed by. Possibly, the wake 120 may be detectable for two or threehours after the submarine has passed. If the submarine is travelling ata speed of eighteen knots, it is 18 to 36 nautical miles away from thepoint at which the wake is detected. A first portion 120a of the wake120 closer to the submarine 118 will have a narrower width than a secondportion 120b of the wake 120 farther from the submarine 118. Thus, asthe aircraft 110 flies overhead and detects the wake 120, if theaircraft 110 flies in a zigzag pattern detecting the width of the wake120, the operator on the aircraft 110 can determine the direction oftravel of the submarine 118. Now the aircraft 110 can continue to fly ina zigzag pattern continuously intersecting the wake 120 to determine thecourse of travel until the aircraft 110 catches up to the submarine 118(it detects at that point the scattering from the hull).

Referring now to FIGS. 1, 1A and 1B, operation of the radar system 10 isexplained. The transmitter 12 is capable of providing a pulsedelectromagnetic signal having a frequency from 1 Hz to 500 Hz. Atechnique for providing a pulsed electromagnetic signal is known andwill not be described in detail. Suffice it to say here that thetransmitted signal 122, for sake of example, has a pulse repetitionperiod of one second and a pulse width of 400 milliseconds. If we assumethat the transmitter 12 is providing a pulsed electromagnetic signalhaving a frequency of 10 Hz, then during one pulse, four periods of thetransmitted electromagnetic signal is emanated. After the transmitter 12has transmitted a pulse of 400 milliseconds, the transmitter stopstransmitting for 600 milliseconds to provide a period for the receiver16 to receive. A more efficient arrangement calls for the transmissionof only one half cycle of the carrier (for instance 1 millisec-longpulse, for a 500 Hz carrier). The pause could be made, as an example, 6millisecond long. After this pause, we could transmit anotherhalf-cycle. However, the polarity would now be reversed.

If we have to operate in shallow waters, we must use short pulses. Anexample will illustrate this case. Let's assume that the carrier is at500 Hz. We transmit a half period of it, which is 1 millisecond long. Weassume also that the depth of the sea is, say, 36 meter. We will becapable of receiving an echo from the bottom, it the system has enoughmargin in Signal-to-Noise ratio to afford 55.5 dB of skin depth losses(at 500 Hz, skin depth is 11.26 meter, and there are 8.68 dB losses foreach skin depth). The delay of the echo from the bottom is about 2millisecond, for a round trip. Because our pulse is 1 millisecond long,we will be capable of keeping available a time interval of 1 millisecondfor the echoes that come back from targets that are in the water column,above sea bottom.

The sinusoidal signal provided within the transmitted pulse (this maycontain several cycles of the carrier) is phase coherent with thetransmitted envelope 122. That is, the sinusoidal signal has a zerodegrees phase at the beginning of a pulse and a 360 degrees phase at theend of the pulse with the number of cycles experienced during the pulsedepending upon the frequency of the sinusoidal signal. The latterreduces the amount of phase fluctuation and improves the accuracy of theradar system 10.

It should be appreciated that the first echo signal to return to theaircraft 110 is the first receive signal 124 which will be the pulse of400 milliseconds duration delayed in time from the transmitted pulse bythe time it takes for a signal to travel in air the distance from theaircraft 110 to the surface 114 of the sea water 112 and return to theaircraft 110 (i.e. two times the height of the aircraft 110 divided by300×10⁶ meters/second). This delay amounts to a few microseconds. Thesecond signal to return to the aircraft 110 is the second receive signal128 which is further delayed after the first receive signal 124 by theamount of time it takes for a signal to travel the distance from thesurface 114 of the sea water 112 to the scattering disturbance 120 andreturn to the surface 114. For most of the time of the duration of eachof the receive pulses, the transmit pulse and the first receive signal124 will mask the second receive signal 128. Only a small portion 128aof the second receive signal 128 will not be hidden which can be usedfor detecting anomalies beneath the surface 114 of the sea water 112.Assuming the disturbance 120 is at a depth of 100 meters and wherein anelectromagnetic signal having a frequency of 10 Hz travels in sea waterat a speed of approximately 5×10³ meters per second (See Table I), thena group delay (identical in this medium to two-way phase delay) ofapproximately 40 milliseconds exists between the first receive signal124 and the second receive signal 128. Thus, the portion 128a of thesecond receive signal 128 will have a duration of 40 milliseconds whichcan then be processed by the receiver 16 for detecting conductivityanomalies. Since the frequency of the electromagnetic signal is 10 Hz,then each cycle has a period of 100 milliseconds so that approximately2/5 ths of a cycle is available for processing by the receiver 16. Itshould be appreciated that the receiver 16 should not be over drivenimmediately before the portion 128a of the second receive signal 128 isfed to the receiver 16. To accommodate such a requirement, the DSP 22(FIG. 1) controls a switch to isolate the receiver 16 from the antenna18 until it is desired for the antenna 18 to be connected to thereceiver 16. This function can also be performed with a softwareapproach, by "blanking-out" the output of the receiver for the entireduration of the radiated burst.

Undisturbed sea water has a certain conductivity profile along thevertical. If the water is stratified, this profile has a maxima andminima. The electromagnetic signal is propagating through the sea water112. As the submarine 118 travels through the sea water 112, thesubmarine 118 induces a complex hydrodynamic phenomenon that includesturbulence, internal waves, eddies, vortices, etc. (i.e. it creates awake), which changes the conductivity of that portion (i.e. thedisturbance 120) of sea water. As the distance from the submarine 118increases, the disturbance 120 will include internal waves and vortexwaves and as the distance further increases, the disturbance 120 willinclude internal waves and vortex eddies. The latter provides a means todetect that the submarine 118 has passed by detecting the electricalconductivity perturbations.

An electromagnetic signal propagating through the sea water 112, havinga certain conductivity, when striking the disturbance 120, having adifferent conductivity, is reflected and scattered and a portion of thereflected signal is reflected toward the aircraft 110. It should beappreciated that an electromagnetic signal propagating in sea water willhave a different velocity of propagation depending upon the frequency ofthe electromagnetic signal. As described hereinabove, a signal having afrequency of 10 Hz will have in sea water a velocity of approximately5×10³ meters per second, whereas a signal having a frequency of 100 Hzwill have a velocity of approximately 15.8×10³ meters per second. Fromthe above discussion, it should be remembered that only the portion 128aof the reflected signal 128 is used for detecting disturbances. If afrequency of 100 Hz is used instead of a frequency of 10 Hz, then thegroup delay (which, in sea water, and at the frequencies of interesthere, is identical to the phase delay, so that the two terms can be usedinterchangeably) will have a duration of 12.6 millisecond. This meansthat this is the amount of time it takes for a signal to travel thedistance from the surface 114 of the sea water 112 to the disturbance120 and return to the surface 114 (assuming a depth of 100 meters).Using a frequency of 100 Hz, only a duration of 12.6 milliseconds isavailable for collecting useful data by the receiver 16. It should benoted, with a frequency of 100 Hz, a period of one cycle has a durationof ten milliseconds. Thus, during the 12.6 milliseconds duration thereceiver 16 is receiving useful information, approximately 1 and 1/4 thof a cycle is available for processing by the receiver 16.

Although the first receive signal 124 is decoupled from the receiver 16by a switch, other sources of unwanted noise (i.e. background clutter)may exist. For example, scatter from surface waves or ripples on thesurface 114 provide unwanted signals to the receiver 16 during thedesired receiving duration. Also perturbations in the conductivity ofthe sea water which are not caused by motion of a vessel provideunwanted background noise which affects the detection capability of theradar system 10. The latter affect the required signal to noise ratio ofthe radar system 10. Considering the operational requirements of theradar system 10, a maximum output power of 35 kilowatts is practicalconsidering the isolation requirements between the receiver 16 and thetransmitter 12. With the above output power limitation, to achieve asatisfactory signal to noise ratio, the radar system 10 must provide anantenna with a strong enough magnetic moment suitable to penetrate thesea water 112, strike a disturbance and provide a reflected signalstrong enough for receiver 16 to process. To achieve the latter, amagnetic moment from approximately 10⁶ to 10⁷ ampere-meters² (Am²) isrequired. Furthermore, wherein a pulsed signal is transmitted and sincethe duration of the usable receive signal is so short, a time constantof the order of approximately one millisecond is desired for the radarsystem 10 to minimize the harmful effects of ringing.

The receiver 16 processes the received signal and provides a signal tothe DSP 22 wherein the signal is digitized and provides a twodimensional array of the received echo intensity versus time. The twodimensional array is fed to the display 24 wherein the two dimensionalarray is displayed to an operator. In the contemplated radar system 10,each frame of the two dimensional array corresponds to a time durationof approximately 1800 seconds, or thirty minutes. Each successive frameoverlaps a preceding frame by 200 seconds, that is if the first frame ofinformation displayed detected echo from the time zero to 1800 seconds,then the second frame of information displays detected echo from thetime 1600 seconds to 3400 seconds. Conventional memory within the DSP 22provides the necessary data storage so that the display 24 may displaythe array of the received echo intensity versus time.

Referring now to FIG. 2, a block diagram of the transmitter 12 is shownto include a waveform generator 30, a switch 32, a power amplifier 34, atuning capacitor 36, a snubber circuit 38 and control circuitry 40. Theoutput of the transmitter 12 is coupled to the antenna 20, via theswitch box 14, as shown. More particularly, the output of thetransmitter 12 provides a pulsed electromagnetic signal preferablyhaving a pulse width of 400 milliseconds and a pulse repetition intervalof one pulse per second. Due to the relatively quick return time of thereturn signal and the short duration of the desired receive signal, apulsed electromagnetic signal with a fast rise and fall time is desired,preferably one millisecond. It should be appreciated that, assuming adepth of 100 meters, an electromagnetic signal having a frequency of 100Hz will typically provide a desired signal for 12.6 milliseconds, afrequency of 10 Hz will typically provide a desired signal for 40milliseconds and a frequency of one Hz will typically provide a desiredsignal for 126 milliseconds. With the latter in mind, it is desirablefor the antenna 20 to have a relatively short time constant. The timeconstant of the antenna 20 is controlled by an inductance component 20aand a resistive component 20b. A technique to reduce the time constantof the antenna 20 is to minimize its inductance. Here, this is achievedwith the use of a solenoidal antenna with a "current sheet" winding aswill be discussed hereinafter in conjunction with FIGS. 4 and 4A. Theantenna 20, although designed with reduced inductance, typicallyincludes an inductance of 11.2 millihenrys and a resistance of 0.2 ohmswhich provides an uncorrected time constant of 11.2/0.2=53 milliseconds.To further reduce the time constant, a technique of preciselycontrolling the timing of the coupling of the electromagnetic signal tothe antenna 20 is utilized. That is the DSP 22 controls the opening andclosing of switch box 14 so that the connection is broken when thecurrent through antenna 20 falls to approximately zero amperes. Sincethe effect of this technique is to reduce the time constant, the use ofthe transmitter 12 is effectively equivalent to providing a syntheticimpedance in series with the antenna 20 without the undesirable effectof increasing power consumption. Thus, the transmitter 12 may bereferred to as a synthetic output impedance circuit.

The waveform generator 30 provides the desired electromagnetic signalhaving a frequency, for example, ten Hz when the desired frequency ofthe output signal is ten Hz. The output signal of the waveform generator30 is controlled by the control circuitry 40 in response to a controlsignal from the DSP 22 (FIG. 1). The output of the waveform generator 30is fed to a switch 32 which is controlled by the control circuitry 40 inresponse to a control signal from the DSP 22 (FIG. 1). The switch 32 isclosed to provide the desired pulse width and open and closed at thedesired pulse repetition frequency to provide the desired pulsedelectromagnetic signal. The switch 32 is closed in a manner to ensurethat the sinusoidal signal provided by the waveform generator 30 has azero degrees phase when the switch is closed to begin a pulse and openedin a manner to ensure the sinusoidal signal has a 360 degrees phase whenthe switch is opened to end a pulse. The latter ensures the sinusoidalsignal is phase coherent with the desired pulsed electromagnetic signal.

The pulsed electromagnetic signal is fed to an input of a poweramplifier 34 to amplify the signal to the desired power output. Theoutput of the amplifier 34 is coupled to a tuning capacitor 36, whichwhen connected to the antenna 20 by the switch box 14, brings theantenna 20 to resonance at the desired frequency as controlled by acontrol signal from the control circuitry 40. Typically, in thepreferred embodiment, the tuning capacitor 36 is set for a value of 4.21millifarads to provide resonance at a frequency of approximately twentyfive Hz and is set for a value of 1.65 millifarads to provide resonanceat a frequency of approximately forty Hz. At frequencies below five Hz,the tuning capacitor is not required. Above forty Hz, the voltage acrossthe tuning capacitor 36 becomes undesirable large, thus it is preferredto operate at frequencies below forty Hz. It should be appreciated thata bias voltage is developed across the tuning capacitor 36, heretypically 1.2 kilovolts and as the switch box 14 selectively couples thetransmitter 12 to the antenna 20, transients or voltage spikes typicallyexperienced when providing a pulsed signal to an inductor (i.e. antenna20) are minimized.

A portion of the output signal is coupled to the control circuitry 40wherein control signals are provided to ensure the waveform generatorand the switch 32 operate in a manner to provide the correct pulsedelectromagnetic signal and to ensure the switch box 14 is opened onlywhen the current of the transmitted electromagnetic signal isapproximately zero amperes. By monitoring the transmittedelectromagnetic signal, the phase and zero crossings of the signal arecontrolled so that the current is disabled from the antenna 20 when thecurrent is exactly zero. At this moment, the voltage across the tuningcapacitor 36 is at a maximum, here approximately 1,000 volts. When thenext pulse is fed to the capacitor 36 and coupled to the antenna 20 bythe switch box 14, the pulse is enhanced by the voltage across thetuning capacitor 36 reducing the effects of transients and ringing thatwould normally be associated when pulsating a large inductance asprovided by the antenna 20.

The transmitter 12 also includes a snubber circuit 38 having a resistor42 and a capacitor 44. The snubber circuit 38 dampens any transients orringing caused by switching the electromagnetic signal to the antenna 20not eliminated by the previously described techniques. Although thetiming of the opening and closing of the switch box 14 is closelycontrolled, there may still be some residual transients and some ringingwhich are eliminated with the snubber circuit 38. Table II shows typicalperformance parameters for a transmitter 12 used in conjunction with theantenna 20.

                  TABLE II                                                        ______________________________________                                        Parameter        1 Hz    5 Hz    25 Hz 40 Hz                                  ______________________________________                                        Antenna Voltage (V-RMS)                                                                        80      142     80    80                                     Antenna Current (A-RMS)                                                                        391     391     391   391                                    Tail Current Decay (MS)                                                                        <10     <10     <10   <10                                    Two-way Phase Delay (MS)                                                                       126     40      25    20                                     Power (KW)       31      31      31    31                                     Power (KVA)      33      55      31    31                                     Tuning Capacitor (MF)                                                                          No      No      4.21  1.65                                   Tuning Capacitor (V-RMS)                                                                       NA      NA      1000  1500                                   Load Disconnect Switch Req'd                                                                   No      No      Yes   Yes                                    Flux Density in Core (KGauss)                                                                  10      10      10    10                                     ______________________________________                                    

Referring now to FIGS. 1 and 1B, received signals are captured by theantenna 18 and coupled to the receiver 16. In accordance with knowntechniques, a switch selectively connects the antenna 18 to theremaining portion of the receiver 16 as controlled by the DSP 22. Thus,during the period the first reflected signal 124 is incident on theantenna 18, the switch disconnects the antenna 18 from the receiver 16.During the period the portion 128a of the second reflected signal 128 isincident on the antenna 18, the switch is closed to connect the antenna18 to the receiver 16 to provide a received signal. The received signalis fed to a phase lock loop. A portion of the transmitted signal fromthe transmitter 12 is also fed to the phase lock loop as a referencesignal. Here the phase lock loop includes a set of nine phase shifterswhich control the amount of phase shift imparted to a signal propagatingtherethrough as controlled by a control signal fed from the DSP 22. Aset of nine mixers interoperate with the set of nine phase shifters inthe phase lock loop to provide nine output signals which are fed to ninecorresponding matched filters. The latter provide an output signal upondetection of an echo signal. The output signal of the matched filters isfed to the DSP 22 wherein the signal is digitized and processed inaccordance with known techniques.

Referring now to FIGS. 3 and 3A, the construction of the antenna 20 isshown. The antenna 20 provides a low impedance and a high magneticmoment. The low impedance is required to ensure a short time constantrequired for the radar system 10 (FIG. 1). The antenna 20 is aHorizontal Magnetic Dipole (HMD) antenna instead of an HorizontalElectric Dipole (HED) antenna. For the frequencies required, an electricdipole antenna in air would require an unrealistic length to achieveresonance, and thus carry a large current. As described above, amagnetic moment of approximately 10⁶ to 10⁷ Am² is required for thecontemplated radar system 10. The antenna 20 is a solenoidal antennahaving a "sheet-like" winding provided by conductive sheet 50 wound in ahelical pattern. The conductive sheet 50 is a copper foil having athickness of ten millimeters and a width of approximately 330millimeters. A dielectric beam 52 is disposed within the winding of theconductive sheet 50 to provide a support structure to maintain themechanical integrity of the antenna 20. The dielectric beam 52 iscomprised of a dielectric material, here wood, and has a square crosssection. A laminated core 54 is disposed between the dielectric beam 52and the winding of conductive sheet 50 to increase the magnetic momentof the antenna 20. The conductive sheet 50 is wrapped around thelaminated core 54 at an angle α, here 13 degrees. Such an angle provides45 turns of the conductive sheet 50 around the laminated core 54 at alength of 15 meters.

The laminated core 54 is comprised of a silicon-iron alloy, here amaterial known as ARMCO-ORIENTED M-6 material manufactured by NationalMaterial Co, of Arnold, Pa. The permeability of the laminated core 54when fabricated from the M-6 material is at least 5000. The antenna 20,as shown, has a square cross-section with a height H and a width W ofapproximately 355 millimeters and a length L of approximately 15 meters.

It should be understood that the magnetic moment of a solenoidal typeantenna is given by P=N·I·A, where P is the magnetic moment in Am², N isequal to the number of turns of the winding, I is the current flowingthrough the winding in Amperes and A is the cross-sectional area of thesolenoid in square-meters. The inductance L of such a solenoidal typeantenna is given by: ##EQU1## where L is the inductance in Henrys and 1is the length of the solenoid in meters. From the above, it is apparentthat if the number of turns N in the winding is increased, the magneticmoment is increased. Unfortunately, the latter also increases theinductance by the square of the number of turns N. Thus, to minimize theinductance, it is desirable to minimize the number of turns of thewinding. In the present antenna 20 using a conductive sheet 50 for thewinding, the magnetic moment is characterized by the equation: P_(s)=k·A·l, where P_(s) is the magnetic moment, A is the cross-sectionalarea of the solenoid in square-meters, 1 is the length of the solenoidin meters and k is the current density of the conductive sheet 50. Fromthe above, k can be expressed as: ##EQU2## The inductance of the antenna20 is given by: ##EQU3## Thus, the current density k of the antenna 20can be increased without increasing the inductance thereof so that ahigh magnetic moment can be achieved without increasing the inductancecaused by increasing the number of turns N.

The laminated core 54 is provided to increase the magnetic moment.Adding the laminated core 54 changes the equation for magnetic momentto: P_(s) =μ·k·A·1, where μ is the permeability of the laminated core54. The equation for inductance is also modified wherein: ##EQU4## Itshould be appreciated that the inductance of the antenna 20 is increasedwith the use of the laminated core 54 as well as increasing the magneticmoment. At an operating frequency of one Hz, the reactance of theantenna 20 is approximately 67.23 milliohms and at a frequency of fiveHz, the reactance of the antenna 20 is approximately 336.15 milliohms.The resistance of the antenna 20 is approximately 200 milliohms. Withthe latter values, at a frequency of one Hz, approximately 35 KVA ofreactance power is required to drive the antenna 20 and at a frequencyof five Hz, approximately 55 KVA of reactance power is required to drivethe antenna 20. Such power requirements are readily realizable in thecontemplated radar system 10.

The laminated core 54 is comprised of lamination of a silicon and ironalloy having a relative permeability of 10,000 and a saturation flux ofapproximately fifteen kilogauss. With a hollow core, the laminated core54 provides suitable permeability while minimizing the weight of theantenna 20. The laminated core 54 has a weight of 7.8 tons/M³ (i.e. 498lb/ft³). The laminated core has a thickness t of approximately 3.5centimeters, although a thickness t of 2.0 or 5.0 centimeters may alsobe suitable. The weight of a 15 meter long laminated core 54 having athickness of 2.0, 3.5 and 5.0 centimeters is approximately 6602 lbs.,11,556 lbs. and 16,509 lbs., respectively. It is noted that, whentrading off weight versus magnetic moment achieved due to a greaterthickness, the 3.5 centimeter thickness was preferred. However, anincrease in the amount of current fed to the antenna 20 will increasethe magnetic moment of the antenna 20.

The antenna 20 is disposed in a dielectric box 56 (a portion of which isshown in FIGS. 3 and 3A) for mounting to the aircraft 110 (FIG. 1) andto protect the conductive sheet 50 from damage. A second differentdielectric box (not shown) can be used as an external container to mountthe dielectric box 56 within to provide additional protection from theenvironment for the antenna 20. As shown in FIG. 1, the antenna 20 isdisposed in the underside of the fuselage of the aircraft 110.Alternatively, the antenna 20 could be mounted in a housing attached tothe aircraft 110 by a towline. If the antenna 20 is mounted in theunderside of the fuselage of the aircraft 110, then shielding isprovided between the antenna 20 and the inside of the aircraft to reducethe electromagnetic field intensity to a level believed to be safe forhuman exposure.

Referring now to FIG. 4, an exemplary detection scene is shown with asubmarine 24 creating a wake 26 which trails behind. An aircraft 110 isshown with the path of the aircraft 110 shown by a dotted line labelled62. The aircraft 110 upon encountering the wake 26, travelled in azigzag pattern crossing the wake 26 multiple times until catching up tothe submarine 24 and passing the submarine 24 as shown. A correspondingplot of the anticipated waveform generated in response to return signalsreceived by the radar system 10 is shown with time plotted along theabscissa and amplitude plotted along the ordinate. At a time t₁, theaircraft encounters the wake 26 and the radar system 10 receives an echosignal which continues until a time t₂ when the aircraft 110 travelsbeyond the wake 26. At a time t₃. the aircraft 110 changes course and ata time t₄ encounters the wake 26. Again an echo signal is received untila time t₅ when the aircraft 110 travels beyond the wake 26. At a timet₆, the aircraft 110 again changes course and at a time t₇ encountersthe wake 26 and receives an echo signal. At a time t₈, the aircraft 110travels beyond the wake 26. It should be noted that as the aircraft 110travels closer to the submarine 24 crossing the wake 26, the duration ofthe echo signal decreases since the wake 26 is getting narrower. Thelatter indicates that the aircraft is travelling in a direction towardthe submarine 24. At a time t₉, the aircraft 110 changes course and, inthis instance, at a time t₁₀ encounters the submarine 24, at a time t₁₁,crosses the center of the submarine 24, and at a time t₁₂, travelsbeyond the submarine 24. It should be noted in most instances, theaircraft 110 would not necessarily cross over directly the submarine,but if such an occurrence does occur then the amplitude of the echosignal received is greater than that of an echo signal received from thewake 26.

Referring now to FIG. 5, an alternative use of the radar system 10mounted on an aircraft 110 is shown. The radar system 10 will emanate anelectromagnetic signal 122 toward a surface 114 of sea water 112. Aportion of the electromagnetic signal 122 is reflected by the surface114 of the sea water 112 as a first reflected signal 124 and a portionof the electromagnetic signal 122 will enter below surface 116 of thesea water 112 as an underwater signal 126. The underwater signal 126will propagate through the sea water 112 until striking a conductivitydisturbance, here an oil plume 160. It should be appreciated that theconductivity of a plume of oil will be vastly different than that of seawater. Upon striking the oil plume 160, a portion of the underwatersignal 126 will be reflected as a second reflected signal 162. Thesecond reflected signal 162 will propagate through the sea water 112until reaching the surface 114 wherein the second reflected signal 162will enter the air and propagate toward the aircraft 110. A portion ofthe second reflected signal 162 will be captured by the radar system 10as a second received signal to be processed as described hereinbefore.

Furthermore, a portion of underwater signal 126 will propagate throughthe oil plume 160 and when reaching the bottom of the oil plume 160where sea water is again encountered, a portion of the underwater signal126 will be reflected as a third reflected signal 164. The thirdreflected signal 164 will propagate through the oil plume 160 and thesea water 112 until reaching the surface 114 wherein the third reflectedsignal 164 will enter the air and propagate toward the aircraft 110. Aportion of the third reflected signal 164 will be captured by the radarsystem 10 as a third received signal to be processed as describedhereinbefore. The radar system 10 processes the second and thirdreceived signals to detect the presence of an under water oil plume. Thesecond received signal determines the depth of the top of the oil plume160 and the third received signal determines the depth of the bottom ofthe oil plume 160. By flying aircraft 110 around the area above the oilplume 160, the radar system 10 can map the shape of the oil plume 160and over time determine the movement of the oil plume 160. From thelatter, the direction and dispersion rate of the oil plume 160 can bedetermined.

It should be appreciated that the antenna 20 would also work well in alow frequency communication system wherein it is desirable tocommunicate from above the water's surface to beneath the water'ssurface. For example, a one way communication signal to a deeplysubmerged submarine, a low data rate emergency communication signal oran underwater rescue beacon signal could be communicated from above thewater's surface to below the water's surface or vice versa.

Having described this invention, it will now be apparent to one of skillin the art that changes may be made without departing from the disclosedconcept. It is felt, therefore, that this invention should not berestricted to its disclosed embodiment, but rather should be limitedonly by the spirit and scope of the appended claims.

What is claimed is:
 1. A radar system for transmitting anelectromagnetic signal into water having a surface from above thesurface of the water to below the surface of the water comprising:(a)means for transmitting a pulsed electromagnetic signal having afrequency less than 500 Hz; (b) means for receiving a reflected signalprovided from the pulsed electromagnetic signal reflected from ananomaly below the surface of the water; (c) means for inhibiting thereceiving means from receiving a reflected signal provided from thepulsed electromagnetic signal reflected from the surface of the water;and (d) means for controlling the transmitting means, the receivingmeans and the inhibiting means.
 2. The radar system as recited in claim1 further comprising a signal processor means for processing saidreflected signals reflected from the anomaly for detecting the presenceof said anomaly.
 3. The radar system as recited in claim 2 wherein thesignal processor means comprises means for detecting a wake created by avessel moving in the water.
 4. The radar system as recited in claim 1wherein the transmitting means comprises a magnetic dipole antenna. 5.The radar system as recited in claim 2 wherein the signal processingmeans comprises means for detecting a plume of oil disposed beneath thesurface of the water.
 6. A method of operating a radar system fordetecting an anomaly beneath the surface of water comprising the stepsof:(a) transmitting a pulsed electromagnetic signal toward the surfaceof the water; (b) inhibiting reception of signals for a length of timecorresponding to the length of time required for the pulsedelectromagnetic signal to travel to and reflected from the surface ofthe water; (c) receiving the signals reflected from an anomaly beneaththe surface of the water; and (d) processing signals reflected from theanomaly beneath the surface of the water to determine the presence ofthe anomaly.
 7. The method of operating a radar system as recited inclaim 6 wherein the step of processing signals further comprises thestep of determining the anomaly is a long lasting wake created by amoving vessel beneath the surface of the water.
 8. The method ofoperating a radar system as recited in claim 6 wherein the step ofprocessing signals further comprises the step of determining the anomalyis a plume of oil disposed beneath the surface of the water.